Adaptive electrode for bi-polar ablation

ABSTRACT

Cardiac ablation is carried out by placing two ablation electrodes on opposite sides of a wall of the heart to generally oppose one another. The effective current transmission area of one of the electrodes is then varied according to the distance between the two electrodes or the thickness of the wall. Sufficient electrical current is transmitted between the two electrodes to achieve transmural ablation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of commonly-assigned U.S. patentapplication Ser. No. 13/971,887, filed Aug. 21, 2013, Now U.S. Pat. No.10,213,248, the entire disclosure of which is incorporated by reference.

FIELD OF THE PRESENT DISCLOSURE

This invention relates to tissue ablation systems. More particularly,this invention relates to improvements in bipolar ablation.

DESCRIPTION OF THE RELATED ART

Cardiac arrhythmias, such as atrial fibrillation, occur when regions ofcardiac tissue abnormally conduct electric signals to adjacent tissue,thereby disrupting the normal cardiac cycle and causing asynchronousrhythm.

Procedures for treating arrhythmia include surgically disrupting theorigin of the signals causing the arrhythmia, as well as disrupting theconducting pathway for such signals. By selectively ablating cardiactissue by application of energy via a catheter, it is sometimes possibleto interrupt or modify the propagation of unwanted electrical signalsfrom one portion of the heart to another. The ablation process destroysthe unwanted electrical pathways by formation of non-conducting lesions.

The Maze procedure is one method of surgical treatment of atrialfibrillation. It involves making a series of incisions in the atria toconstruct a “maze” of scar tissue that acts as a barrier to the erraticelectronic impulses associated with atrial fibrillation, allowing onlythose following the correct path to the heart to get through. Althoughhighly successful, the Maze procedure is technically difficult andrequires stopping the heart and placing the patient on a heart-lungmachine.

A known difficulty in the use of radiofrequency energy for cardiactissue ablation is controlling local heating of tissue. There aretradeoffs between the desire to create a sufficiently large lesion toeffectively ablate an abnormal tissue focus, or block an aberrantconduction pattern, and the undesirable effects of excessive localheating. If the radiofrequency device creates too small a lesion, thenthe medical procedure could be less effective, or could require too muchtime. On the other hand, if tissues are heated excessively then therecould be local charring effects due to overheating. Such overheatedareas can develop high impedance, and may form a functional barrier tothe passage of heat. The use of slower heating provides better controlof the ablation, but unduly prolongs the procedure.

SUMMARY OF THE INVENTION

Bipolar radiofrequency ablation is one approach to simplifying theablation procedure. Instead of using surgical incisions, doctors createa lesion in the heart by passing radiofrequency current through twoelectrodes located on opposite sides of the heart wall or septum,causing a transmural lesion 1-2 mm in width. The procedure does notrequire stopping the heart, and each lesion takes 9 seconds to complete,as opposed to 5-10 minutes per lesion using the Maze procedure.

Although transmural ablation in the left ventricle may be appropriatefor treatment of arrhythmias such as refractory ventricular tachycardia,it is not feasible using current ablation catheters and methods. Theeffective ablation zone generated in the myocardium using an irrigatedablation catheter extends only about 5 mm beneath the contacting surfaceof the ablation electrode. As the left ventricle thickness may be atleast 15 mm, it is apparent that even ablating from both sides of theventricle fails to achieve the objective.

Embodiments of the present invention provide a catheter and method foradaptively shaping a lesion and effectively controlling its deptheffectively to at least 15 mm.

There is provided according to embodiments of the invention a method ofablation, which is carried out by placing a first ablation electrode ofa first probe at a first side of a wall of the heart of a livingsubject, placing a second ablation electrode of a second probe with at asecond side of the wall to oppose the first ablation electrode, varyingan effective current transmission area of the second ablation electrode,and flowing sufficient electrical current between the first ablationelectrode and the effective current transmission area of the secondablation electrode to ablate the wall.

According to still another aspect of the method, flowing sufficientelectrical current is performed while at least one of the first andsecond ablation electrodes is in contact with the wall.

According to another aspect of the method, flowing sufficient electricalcurrent is performed while at least one of the first and second ablationelectrodes is within 2 mm of the wall.

According to an additional aspect of the method, varying an effectivecurrent transmission area is performed responsively to the distancebetween the first and second ablation electrode.

According to one aspect of the method, varying an effective currenttransmission area is performed responsively to a thickness of the wall.

According to yet another aspect of the method, the second ablationelectrode includes a plurality of segments, the segments is electricallyinsulated from one another, and each of the segments is switchablyconnectable to a source of the electrical current.

According to a further aspect of the method, the segments compriseconcentric circles.

According to yet another aspect of the method, the segments are arrangedin a spiral.

According to one aspect of the method, borders of the segments comprisetriangles.

According to an additional aspect of the method, the triangles aresimilar triangles has a common geometric center.

According to still another aspect of the method, the effective currenttransmission area of the second ablation electrode is between 2 and 4times as large as the effective current transmission area of the firstablation electrode.

According to yet another aspect of the method, the effective currenttransmission area of the second ablation electrode is between 3 and 4times as large as the effective current transmission area of the firstablation electrode.

In another aspect of the method, the second ablation electrode comprisesan electroconductive film and electrical signals are applied to the filmto cause shape shifting thereof.

In yet another aspect of the method the second ablation electrodecomprises an electroconductive film having a shape memory, and themethod includes unfolding the film for deployment thereof and refoldingthe film for disengagement thereof with the subject.

According to an additional aspect of the method, the second ablationelectrode is formed of a carbon-nanofiber, oxidized carbon-nanofiber, orcarbon black-filled conductive shape-memory polyurethane composite.

There is further provided according to embodiments of the invention anablation apparatus, including a first flexible probe adapted forinsertion into the heart of a living subject and a first ablationelectrode disposed at the distal segment of the probe, the probe adaptedto be brought to a target tissue at a first side of a wall of the heart.The apparatus further includes a second ablation electrode adapted to bebrought to a second side of the wall to oppose the first ablationelectrode, the second ablation electrode including a plurality ofsegments that are electrically insulated from one another, and a powergenerator connectable to the first ablation electrode and switchablyconnectable to selected ones of the segments of the second ablationelectrode for passing electric current between the first ablationelectrode and the selected segments of the second ablation electrode.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

For a better understanding of the present invention, reference is madeto the detailed description of the invention, by way of example, whichis to be read in conjunction with the following drawings, wherein likeelements are given like reference numerals, and wherein:

FIG. 1 is a pictorial illustration of a system for performing ablativeprocedures on a heart of a living subject, which is constructed andoperative in accordance with an embodiment of the invention;

FIG. 2 is a schematic diagram illustrating a simulation environment forevaluation of transmural ablation, in accordance with an embodiment ofthe invention;

FIG. 3 is a display of the temperature field in an experiment conductedusing the simulation environment shown in FIG. 2, in accordance with anembodiment of the invention;

FIG. 4 is a graph indicating the relationship between the duration ofcurrent application and maximum temperature in the experimentillustrated in FIG. 3, in accordance with an embodiment of theinvention;

FIG. 5 is a display of the temperature field in an experiment conductedusing the simulation environment shown in FIG. 2, in accordance with anembodiment of the invention;

FIG. 6 is a graph indicating the relationship between the duration ofcurrent application and maximum temperature in the experimentillustrated in FIG. 5, in accordance with an alternate embodiment of theinvention;

FIG. 7 is a display of the temperature field in an experiment conductedusing the simulation environment shown in FIG. 2, in accordance with analternate embodiment of the invention;

FIG. 8 is a graph indicating the relationship between the duration ofcurrent application and maximum temperature in the experimentillustrated in FIG. 7, in accordance with an alternate embodiment of theinvention;

FIG. 9 is a schematic diagram of a patch electrode, shown retracted in afolded configuration within the lumen of an insertion tool, inaccordance with an alternate embodiment of the invention;

FIG. 10 is a schematic diagram of a patch electrode, which has beenextended beyond the insertion tool shown in FIG. 9 and unfolded fordeployment in accordance with an embodiment of the invention; and

FIG. 11 is a schematic diagram of a patch electrode, which has beenextended beyond the insertion tool shown in FIG. 9 and unfolded fordeployment in accordance with an alternate embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the various principles ofthe present invention. It will be apparent to one skilled in the art,however, that not all these details are necessarily always needed forpracticing the present invention. In this instance, well-known circuits,control logic, and the details of computer program instructions forconventional algorithms and processes have not been shown in detail inorder not to obscure the general concepts unnecessarily.

Aspects of the present invention may be embodied in software programmingcode, which is typically maintained in permanent storage, such as acomputer readable medium. In a client/server environment, such softwareprogramming code may be stored on a client or a server. The softwareprogramming code may be embodied on any of a variety of knownnon-transitory media for use with a data processing system, such as adiskette, hard drive, electronic media or CD-ROM. The code may bedistributed on such media, or may be distributed to users from thememory or storage of one computer system over a network of some type tostorage devices on other computer systems for use by users of such othersystems.

Definitions

The term “effective current transmission area”, when applied herein toan electrode, refers to an area of the electrode, which is operationallycapable of supporting passage of an electric current through theelectrode, e.g., between the electrode and a target to which theelectrode is in contact.

System Description

Turning now to the drawings, reference is initially made to FIG. 1,which is a pictorial illustration of a system 10 for performing ablativeprocedures on a heart 12 of a living subject, which is constructed andoperative in accordance with an embodiment of the invention. The system10 comprises a catheter 14, which is percutaneously inserted by anoperator 16 through the patient's vascular system into a chamber orvascular structure of the heart 12. The operator 16, who is typically aphysician, brings the catheter's distal tip 18 into contact with theheart wall at an ablation target site. Optionally, Electrical activationmaps may then be prepared, according to the methods disclosed in U.S.Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat.No. 6,892,091, whose disclosures are herein incorporated by reference.One commercial product embodying elements of the system 10 is availableas the CARTO® 3 System, available from Biosense Webster, Inc., 3333Diamond Canyon Road, Diamond Bar, Calif. 91765.

Areas determined to be abnormal, for example by evaluation of theelectrical activation maps, can be ablated by application of thermalenergy, e.g., by passage of radiofrequency electrical current throughwires in the catheter to one or more electrodes at or near the distaltip 18, which apply the radiofrequency energy to the myocardium. Theenergy is absorbed in the tissue, heating it to a point (typically about50.degree. C.) at which it permanently loses its electricalexcitability. When successful, this procedure creates non-conductinglesions in the cardiac tissue, which disrupt the abnormal electricalpathway causing the arrhythmia. The principles of the invention can beapplied to different heart chambers to treat many different cardiacarrhythmias.

The catheter 14 typically comprises a handle 20, an ablation electrode32 at or near its distal extremity, and having suitable controls on thehandle to enable the operator 16 to steer, position and orient thedistal portion of the catheter as desired for the ablation. To aid theoperator 16, the distal portion of the catheter 14 contains positionsensors (not shown) that provide signals to a positioning processor 22,located in a console 24.

A second probe, epicardial catheter 27 is connected to the console 24,and features an ablation element 41 at its working end. The ablationelement 41 is positioned to oppose the ablation electrode 32 with targettissue 43 of the heart 12 therebetween. The ablation electrode 32 isconnected via cable 34 to the console 24. The catheter 27 can be placed,for example, using the PerDUCER® Access Device, available from ComedicusInc., 3989 Central Avenue N.E., Suite 610, Columbia Heights, Minn.55421.

While the second probe is shown as an epicardial catheter in FIG. 1,this is not necessarily the case. For example, if it were required toablate interventricular septum 45 the second probe would be introducedinto the chamber of right ventricle 47 and the catheter 14 would contactthe interventricular septum 45 from within the chamber of left ventricle49 so as to oppose the second probe. The ablation element 41, shownschematically in FIG. 1, is described in further detail hereinbelow.

Ablation energy and electrical signals can be conveyed to and from theheart 12 through the ablation electrodes 32, 41. For example, pacingsignals and other control signals may be conveyed from the console 24through the cable 34 and the ablation electrode 32 to the heart 12.Sensing electrodes 31, 33, also connected to the console 24 are disposednear the ablation electrode 32 and have connections to the cable 34.

Wire connections 35 link the console 24 with body surface electrodes 30and other components of a positioning sub-system. The ablation electrode32 and the body surface electrodes 30 may be used to measure tissueimpedance at the ablation site as taught in U.S. Pat. No. 7,536,218,issued to Govari et al., which is herein incorporated by reference. Atemperature sensor (not shown), typically a thermocouple or thermistor,may be mounted on or near the ablation electrode 32.

The console 24 typically contains one or more ablation power generators25. The catheters 14, 27 may be adapted to conduct ablative energy tothe heart using any known ablation technique, e.g., radiofrequencyenergy, ultrasound energy, and laser-produced light energy. Such methodsare disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924,and 7,156,816, which are herein incorporated by reference.

The positioning processor 22 is an element of a positioning subsystem inthe system 10 that measures location and orientation coordinates of thecatheters 14, 27.

In one embodiment, the positioning subsystem comprises a magneticposition tracking arrangement that determines the position andorientation of the catheters 14, 27 by generating magnetic fields in apredefined working volume and sensing these fields at the catheter,using field generating coils 28. The positioning subsystem may employimpedance measurement, as taught, for example in U.S. Pat. No.7,756,576, which is hereby incorporated by reference, and in theabove-noted U.S. Pat. No. 7,536,218.

As noted above, the catheters 14, 27 are coupled to the console 24,which enables the operator 16 to observe and regulate their functions.Console 24 includes a processor, preferably a computer with appropriatesignal processing circuits. The processor is coupled to drive a monitor29. The signal processing circuits typically receive, amplify, filterand digitize signals from the catheters 14, 27, including signalsgenerated by the above-noted sensors and a plurality of location sensingelectrodes (not shown) located distally in the catheters 14, 27. Thedigitized signals are received via cable 38 and used by the console 24and the positioning system to compute the position and orientation ofthe catheters 14, 27 and to analyze the electrical signals from theelectrodes.

Typically, the system 10 includes other elements, which are not shown inthe figures for the sake of simplicity. For example, the system 10 mayinclude an electrocardiogram (ECG) monitor, coupled to receive signalsfrom one or more body surface electrodes, so as to provide an ECGsynchronization signal to the console 24. As mentioned above, the system10 typically also includes a reference position sensor, either on anexternally-applied reference patch attached to the exterior of thesubject's body, or on an internally-placed catheter, which is insertedinto the heart 12 maintained in a fixed position relative to the heart12. Conventional pumps and lines for circulating liquids through thecatheters 14, 27 for cooling the ablation site are provided.

Transmural Ablation

Reference is now made to FIG. 2, which is a schematic diagramillustrating a simulation environment 51 for evaluation of transmuralablation, in accordance with an embodiment of the invention. In thesimulation, myocardium 53 has a tip electrode 55 of a conventionalablation catheter 57 applied to endocardium 59. An ablation patch 61,whose effective current transmission area is larger than that of theelectrode 55, typically by a factor of between 2 and 4, and preferably 3and 4 is applied to epicardium 63. The area 65 adjacent the endocardium59 is simulates a thermally diffusive (.about.30 [W/m/K]) fluid 65. Theepicardium 63 is simulated as bathed in fluid 67, which representsordinary blood. The arrangement of FIG. 2 provides a simple temperatureregulation mechanism in for the patch 61 that is equivalent to coolingusing irrigation fluid to a temperature of 30.degree. C.

Example 1

Reference is now made to FIG. 3, which is a display of the temperaturefield in an experiment conducted using the simulation environment 51(FIG. 2) in accordance with an embodiment of the invention. In thisexample, catheter diameter was 2.5 mm; the patch diameter was four timesas large as the catheter diameter (4*2.5 mm); the RF current was 0.35 A;and the tissue thickness was 15 mm. The thermal gradient of heated zone69 after 60 sec of ablation can be appreciated with reference to key 71.The 55.degree. C. isotherm is shown emphasized for convenience.

It should be noted that while tissue thickness was used in this and thefollowing examples as the interelectrode distance, contact between theelectrodes and the tissue is not essential. The techniques describedherein are effective, even when there is a gap of about 2 mm between theelectrodes and the tissue.

In this case ablation would occur within the 55.degree. C. isotherm. Itis evident that this isotherm is transmural. As noted above, it isdesirable to keeping the maximal temperature small enough to preventsteam-pops and charring.

Varying the patch diameter while holding the catheter electrode diameterconstant controls the current density on both sides of the myocardium.The ratio of the patch diameter to the catheter diameter is adjusted tooptimally shape the 55.degree. C. isotherm according to the actualmyocardial thickness.

Too small a ratio will cause the peak temperature to rise too much onboth sides. However, attempting to avoid this by lowering the currentwould result in with two smaller lesions that are not transmural.Increasing the diameter of the patch will lower the temperature at thepatch and assure that the ablation is transmural.

Reference is now made to FIG. 4, which is a graph 73 similar to FIG. 3indicating the relationship between the duration of current applicationand maximum temperature in the example of FIG. 3. A temperature of80.degree. C. is achieved in about 10 seconds.

Example 2

Reference is now made to FIG. 5, which is another display of thetemperature field in an experiment conducted using the simulationenvironment 51 (FIG. 2) after 60 sec of ablation using 0.3 Amp inaccordance with an embodiment of the invention. In this example, thecatheter diameter was 2.5 mm; the patch diameter was (3*2.5) mm; thecurrent 0.3 A; and the tissue thickness 10 mm. When compared to FIG. 3,it is apparent from the 55.degree. C. isotherm that a wider, moreuniform effective transmural ablation temperature has been achieved.

Reference is now made to FIG. 6, which is a graph 75 similar to FIG. 4,indicating the relationship between the duration of current applicationand maximum temperature. A temperature of 80.degree. C. is achieved inabout 20 seconds, taking twice as much time as in the graph 73 (FIG. 4).

Example 3

Reference is now made to FIG. 7, which is another display of thetemperature field in an experiment conducted using the simulationenvironment 51 (FIG. 2) in accordance with an embodiment of theinvention. In this example, the catheter diameter was 2.5 mm; the patchdiameter (3*2.5) mm; the current 0.25 A; and the tissue thickness 7 mm.

Reference is now made to FIG. 8, which is a graph 77 similar to FIG. 3,indicating the relationship between the duration of current applicationand maximum temperature in the example of FIG. 7.

Reference is now made to FIG. 9, which is a schematic diagram of a patchelectrode 79, shown retracted in a folded configuration within the lumenof an insertion tool 81, in accordance with an embodiment of theinvention. The electrode 79 can be advanced beyond the tool 81 usingcontrol wires 83, which may also serve to communicate radiofrequencycurrent and electrical signals to and from the electrode 79.

The electrode 79 is composed of a thin electroconductive film or sheetthat may have a shape memory, and is capable of shape shifting,optionally under control of electrical signals. In any case, theelectrode 79 is able to unfold when extended during the medicalprocedure and to resume its folded configuration, so that it can beretracted into the lumen of the tool 81. For example, carbon-nanofiber,oxidized carbon-nanofiber, or carbon black-filled, conductiveshape-memory polyurethane composites may be used to construct theelectrode 79.

First Alternate Embodiment

Reference is now made to FIG. 10, which is a schematic diagram of apatch electrode 85, which has been extended beyond the tool 81 andunfolded for deployment in accordance with an embodiment of theinvention. The electrode 85 is introduced via the tool 81 as describedabove with reference to FIG. 9. The electrode 85 is divided into aplurality of concentric circular segments 87, 89, 91, 93, which areelectrically insulated from one another. Wires 95 lead from the segments87, 89, 91, 93 to a series of switches 97, which can connect thesegments individually or collectively to the console 24 and one of theablation power generators 25 (FIG. 1) via a cable 99. Additionally oralternatively, the cable 99 may conduct electrical signals from theelectrode 85 to the console 24 (FIG. 1) and control signals from theconsole 24 to the electrode 85. As different ones of the segments 87,89, 91, 93 are switched in and out of electrical communication with theconsole 24, the effective current transmission area of the electrode 85relating to the target tissue 43 (FIG. 1) may be varied according torequirements of the ablation procedure, as discussed above.

Second Alternate Embodiment

Reference is now made to FIG. 11, which is a schematic diagram of apatch electrode 101, which is shown extended beyond the tool 81 andunfolded for deployment in accordance with an embodiment of theinvention. The construction is generally similar to the embodiment ofFIG. 10, however the sheet has a plurality of generally elongated,curved bands demarcated by a continuous insulation line 103 that followsa spiral course from a central point 105 outwardly toward the edge ofthe electrode 101. The bands are further segmented by transverseinsulation lines 107, 109, 111, 113. The segments are attached to wires115, which lead to the cable 99 via the switches 97.

While four segments are shown in the examples of FIG. 10 and FIG. 11,any number of segments may be provided to achieve a desired granularityin the size adjustment of the effective area of the electrodes 85, 101.

Other segmented geometric arrangements for a patch electrode arepossible, for example a series of segments whose borders describetriangles, e.g., similar triangles having a common geometric center. Itis only necessary that the effective current transmission area of thepatch electrode exceed that of the opposing catheter electrode. In anycase, appropriate selection of the segments optimizes the ratio betweenthe effective current transmission area of the patch electrode and thecatheter electrode.

Operation

Prior to a medical procedure, a database of optimum power settings andratios of the effective current transmission areas of the electrodes isprepared for different inter-electrode distances, e.g., using theabove-described simulation or experimentally using animal tissues.

When the electrodes are in position, the inter-electrode distance isdetermined, e.g., by the location sensing facilities of the CARTOsystem. The ablation settings may then be established automatically byswitching in an appropriate number of segments of the patch electrode tocreate a desired ratio of the effective current transmission areas, andestablishing an appropriate power output for the RF generator.Alternatively, the settings may be automatically determined andpresented as recommendations to the operator who may approve or modifythem.

It will be appreciated by persons skilled in the art that the presentinvention is not limited to what has been particularly shown anddescribed hereinabove. Rather, the scope of the present inventionincludes both combinations and sub-combinations of the various featuresdescribed hereinabove, as well as variations and modifications thereofthat are not in the prior art, which would occur to persons skilled inthe art upon reading the foregoing description.

What is claimed is:
 1. An ablation apparatus, comprising: a firstflexible probe adapted for insertion into a heart of a living subjectand having a distal segment and a first ablation electrode disposed atthe distal segment to be brought to a target tissue at a first side of awall of the heart; a second flexible probe adapted for insertion intothe living subject and having a distal end and a second ablationelectrode disposed at the distal end, the second ablation electrode tobe brought to an opposing second side of the wall to oppose the firstablation electrode, the second ablation electrode comprising a pluralityof segments, the segments being arranged in a spiral and electricallyinsulated from one another; and a power generator connectable to thefirst ablation electrode and switchably connectable to selected ones ofthe segments of the second ablation electrode for passing electriccurrent from the first ablation electrode and through the target tissueand for passing electric current from the selected segments of thesecond ablation electrode and through the tissue, the first ablationelectrode having a fixed current transmission area and the secondablation electrode having a variable current transmission area, thevariable current transmission area being selected to form a transmurallesion between the first ablation electrode and the second ablationelectrode.
 2. The apparatus according to claim 1, wherein the effectivecurrent transmission area of the second ablation electrode is between 2and 4 times as large as the effective current transmission area of thefirst ablation electrode.
 3. The apparatus according to claim 1, whereinthe effective current transmission area of the second ablation electrodeis between 3 and 4 times as large as the effective current transmissionarea of the first ablation electrode.
 4. The apparatus according toclaim 1, wherein the second ablation electrode comprises anelectroconductive film that changes shape upon application of electricalsignals thereon.
 5. The apparatus according to claim 1, wherein thesecond ablation electrode comprises an electroconductive film that isconfigured to unfold when extended from a delivery device at a treatmentsite and is configured to fold into the delivery device when the secondablation electrode is retracted from the treatment site.
 6. Theapparatus according to claim 1, wherein the second ablation electrodecomprises a carbon-nanofiber, oxidized carbon-nanofiber, or carbonblack-filled conductive shape-memory polyurethane composite.